Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined number of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or the air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
In the solid oxide fuel cell, the operating temperature is significantly high, about 800° C., and an oxidizing gas and a reducing gas are used. Therefore, it is desirable that the gasket for the solid oxide fuel cell has heat resistance, and can be used suitably in the oxidizing atmosphere and the reducing atmosphere. Further, the gasket needs to have malleability and flexibility for maintaining the desired sealing performance.
In view of the above, Japanese Laid-Open Patent Publication No. 7-57748 (hereinafter referred to as the “first conventional technique”) discloses a gasket member for use in high temperature. The gasket member is formed by mixing ceramic fiber and glass having a high melting point into a sheet-like shape. According to the disclosure of the first conventional technique, sufficient gas sealing performance is maintained at high temperature in the range of 1000° C. or more, and improvement in durability is expected.
Further, Japanese Laid-Open Patent Publication No. 10-12252 (hereinafter referred to as the “second conventional technique”) discloses a seal member made of a sintered body of raw powder chiefly containing oxide powder having the average grain size of 0.5 μm or less and a melting point higher than the operating temperature of the fuel cell. The seal member is used at the seal portion for sealing the end of the interface between a power generation cell and a separator. According to the disclosure of the second conventional technique, the seal member can keep its solid state at the operating temperature of the fuel cell, and the seal member is chemically stable in the oxidizing atmosphere and the reducing atmosphere.
However, in the first conventional technique, if the gasket member is used for a long period of time, it is likely that the high melting point glass is degraded due to repetition of expansion and contraction. As a result, the high melting point glass may be shattered, and the fiber or the like may be shattered from the ceramic fiber due to degradation. Thus, the fuel cell is degraded undesirably.
In the second conventional technique, since the seal member does not have any flexibility, the desired sealing performance may not be achieved. Further, the material of the seal portion is the same as the material of the electrolyte. That is, the material of the seal member for the seal portion is limited to the material of the electrolyte. Thus, the seal member is not suitable for general use. Since the seal member is fragile, the seal member cannot be used at the position where a tightening force is applied.